It is known that the position of a mobile telephone is determined by an access point and the determined position is sent to the telephone. In a method used in a mobile telephone system and a PHS (Personal Handyphone System), the position of a telephone is determined based on the position of an access point which is closest to the telephone. The accuracy or resolution of the positioning is about 100 to 200 m (meters) in the PHS, and is about 800 m or more in the mobile telephone.
In another method used in the PHS, the position of a mobile telephone is determined by a plurality of access points in accordance with respective levels of RF signals received from the telephone. The positioning accuracy is about 40 to 70 m.
It is known that the position of a mobile wireless terminal is determined by the triangulation method based on the propagation times of RF signals from a plurality of access points to the terminal. The positioning accuracy is about 10 to 20 m.
The methods for determining a position in accordance with the levels or propagation times of an RF signal may be effective in a small area where there is no obstacle. In general, however, the radio environment changes easily due to, for example, the influence of multi-paths caused by a building, a wall or the like. In the methods, therefore, detected values exhibit wide variations depending on a device or terminal, and hence the positioning accuracy is low.
A technique of determining the position of a mobile wireless terminal by using the GPS (Global Positioning System) is known. In a method using only the GPS, the positioning accuracy is about 30 to 100 m. In another method using the GPS together with an auxiliary system, a position of a mobile wireless terminal estimated by the GPS is corrected in accordance with information of the position which is measured by a reference station. The positioning accuracy is about 5 to 50 m. In these methods, the signal processing is complex, and the accuracy may be possibly lowered at an arbitrary time in accordance with the intention of the organization managing the GPS satellites. Inside a house or a valley between buildings, the positioning accuracy of the GPS is lowered.
PCT publication WO 95/04943 laid-open by Gernar on Feb. 16, 1995, which claims convention priority of U.S. patent Ser. No. 08/101,945 filed on Aug. 4, 1993, discloses a mono-pulse azimuth radar system for tracking an automotive vehicle. In the system, the position of a preceding vehicle is determined in accordance with the distance and angle of the preceding vehicle relative to the own vehicle. In the system, however, the position of a mobile terminal is not determined.
Japanese Patent Publication No. HEI 8-86864 (A) laid-open by Kago on Apr. 2, 1996 discloses an ETC (Electronic Toll Collection) system in which one lane is divided into four areas, received signals from four antennas of different directionalities are combined with one another to detect the presence or the absence of an incoming vehicle in each of the areas. In this system, in order to perform the detection at high accuracy in a larger range, many small areas and many antennas for the respective areas are required.
Japanese Patent Publication No. HEI 6-222124 (A) laid-open by Tamaoki et al. on Aug. 12, 1994 discloses a technique in which a directional antenna is used, and an actual intensity pattern of a received RF signal transmitted from a mobile unit is matched or compared with predetermined intensity patterns of received signals to determine the position of the mobile unit. The antenna has a relatively large size, and an apparatus of a large size is necessary for controlling the directionality of the antenna.
It is an object of the invention to determine more correctly the position of a mobile wireless terminal in a relatively narrow area.
It is another object of the invention to provide a configuration, which is simpler in structure and smaller in size, for determining the position of a mobile wireless terminal.